Icy satellites harboring subsurface oceans have become major targets in planetary science, especially in the context of habitability, as they may provide the key ingredients for life beneath their icy shells : liquid water, energy sources and chemically reactive compounds. Among them, Enceladus is particularly compelling because it is not only an « ocean world », but also currently active. Indeed, the Cassini-Huygens mission (NASA/ESA/ASI) revealed that material from its interior is continuously ejected from Enceladus’ South Pole in the form of plumes. Therefore, Enceladus offers a unique connection between the subsurface ocean, the ice shell, the surface and Saturn’s environment. Understanding the properties of its surface and shallow subsurface is thus essential, both for interpreting its ongoing activity and for preparing future exploration missions, including lander missions.

Most remote sensing observations of icy surfaces rely on ultraviolet, optical, or infrared techniques. These observations provide valuable constraints on the surface composition, ice grain size, crystallinity and temperature. However, they primarily probe only the uppermost surface (typically µm to mm). In contrast, microwave observation – through radar in active mode and radiometry in passive mode – can penetrate deeper layers, depending on wavelength as well as on the absorption and scattering properties of the icy regolith. Consequently, Cassini RADAR observations offer a unique opportunity to investigate Enceladus beneath its surface, and to constrain key physical properties such as porosity, roughness, grain size, thermal gradients and subsurface layering.

This thesis focuses on the analysis of two sets of unique data collected by the Cassini RADAR during a flyby (called E16) of Enceladus in November 2011. The first dataset consists of a Synthetic Aperture Radar (SAR) image and high-resolution radiometry observations of the South Polar Terrain (SPT) of Enceladus acquired while the spacecraft was at its closest approach. The second dataset consists of four radiometry segments acquired during the E16 outbound leg at mid-latitudes (including part of the Leading Hemisphere Terrain, LHT), with a moderate spatial resolution.

Both datasets show « anomalous » regions on Enceladus. The first dataset shows that anomalously bright terrains in radar and radiometrically anomalously warm regions in radiometry exist on SPT. Second datasets show a large scale radiometrical colder anomaly on LHT in comparison with the typical Trailing Hemisphere Terrain (THT). To investigate these anomalous regions, this work combines a thermal model with a microwave radiative transfer model able to simulate both active and passive microwave observations.

By comparing simulations with the observations, this thesis brings constrains on the structural, chemico-physical and thermal state of surface and subsurface at the SPT. The results point that the regolith is characterized by relatively large scattering structures (>500 µm), limiting radar sensitivity to the upper few meters. They also indicate that the regolith purity and porosity vary with local geology and between the different regions within the swath. The anomalously radar bright regions are covered with pure and highly porous water ice. Radiometrically anomalously warm regions observed over parts of the swath can only be explained by the presence of an ocean at shallow subsurface (2-5 km deep), associated with enhanced heat loss (up to 900 mW/m^2). These results have important implications for future missions to Enceladus, particularly for potential lander missions targeting its south polar region.

Similarly, second datasets are compared with simulations by varying the properties between LHT and THT. Analysis show that anomalously colder LHT can be explained by either higher porosity, or larger grain size than that on THT. Further, the depth of such anomaly is constrained which sheds light on the evolutionary history of Enceladus.

More broadly, the combined thermal and radiative transfer modeling framework developed here provides a robust approach for interpreting microwave observations and relating them to the underlying geophysical properties of icy worlds.

La soutenance sera en anglais.

Location
Sorbonne Université – Jussieu Campus La Salle de Conférence TEB, Tour 46/45 4 Place Jussieu, 75005 Paris, France

Visio
https://zoom.us/j/94952034754

Zoom Passcode – 882216

Valérie CIARLETTI – Présidente (Professeure, UVSQ)

Cécile FERRARI – Rapportrice (Professeure, Université Paris Cité)

Gabriel TOBIE – Rapporteur (DR-CNRS, Université de Nantes)

Marion LEDUC-LEBALLEUR – Examinatrice (Scientist, IFAC-CNR)

Sriram S. BHIRAVARASU – Examinateur (Scientist, Space Applications Centre, ISRO)

Alice LE GALL – Directrice de thèse (Chaire Professeur Senior, Sorbonne Université)

Frédéric SCHMIDT – Codirecteur de thèse (Professeur, Université Paris-Saclay)